In case of a material or a small heating target which has a very small radiation ratio such as aluminum or ceramics, it is difficult to remove heat from such a heating target even though the radiation heat is given thereto or uniformly give heat thereto, and hence this kind of target is heated based on convection heat transfer. Since the velocity of a gas current controls the heat transfer in the convection heat transfer, there has been proposed a gas recirculating furnace for forcibly recirculating a gas current in the furnace. For example, a gas recirculating furnace such as shown in FIG. 11 is used for heating the heating target. This gas recirculating furnace is of a batch type and comprises an out-of-furnace circulating path 106 which connects a combustion chamber 102 with an exhaust chamber 103 on both side walls of a furnace 101 and has a hot blast circulating fan 104 and a duct 105 between these chambers. This gas recirculating furnace forms a recirculating gas current which partially takes out combustion exhaust gas from the exhaust chamber 103 and returns it into the combustion chamber 102. In case of this gas recirculating furnace, the gas current heated by the flame in the combustion chamber 102 passes through the inside of the furnace 107 in a direction orthogonal to the direction along which the heating target is carried and flows into the exhaust chamber 103 while heating the heating target W. Further, a part of the recirculating gas current led into the exhaust chamber 103 is exhausted, and the remaining part of the same is led into a duct (circulating path) 105 to be forcibly recirculated.
In addition, as shown in FIG. 12, in case of a continuous gas recirculating furnace constituting a plurality of zones 107a, . . . , 107e, a combustion chamber 102 and an exhaust chamber 103 such as shown in FIG. 11 are provided on the both side walls of the furnace body 101 although not illustrated in FIG. 12 so that the recirculating gas current crossing the inside of the furnace be formed in each zone. An exhaust opening 110 is formed to the zone 107a adjacent to an entry opening for the heating target 108 in order to collect combustion gas which is generated in the respective zones 107a, . . . , 107e and used for increasing the heat of the gas current and exhaust the collected gas from one position.
However, since the energy for causing the strong recirculating current depends on a flow quantity of the gas current and a pressure, the obtainable strong recirculating current is limited in the conventional gas recirculating furnace for performing forcible recirculation while maintaining the hot blast. That is, the pressure is proportional to a square of a flow velocity, and hence the pressure must be gained in proportion to a square of a flow velocity when increasing the flow velocity. However, increasing the pressure in proportion to the square also extremely increases the power of the circulating fan 106, and the discharge pressure can not be increased, thereby limiting a quantity of recirculation. In other words, it is hard to form the large strong recirculating current. Moreover, a high-temperature hot blast is a target, a heat-resisting blade or fan shaft must be cooled down and failures may be likely to occur. Therefore, a fan which can resist the high-temperature hot blast did not exist and recirculating the high-temperature hot blast was difficult in the prior art. Thus, in the prior art gas recirculating furnace, the limit of the temperature of gas which can recirculate outside the furnace is approximately 650.degree. C., and the heat transfer efficiency can not be improved by increasing a temperature in the furnace. Accordingly, the quantity of heat transfer can not be increased. A heating process time becomes long in the batch type gas recirculating furnace, while a length of the furnace increases in the continuous gas recirculating furnace.
When using a burner as a heat source, realizing the high flow velocity of the gas current having a relative low temperature of not more than about 650.degree. C. may lower the temperature of the flame, and the flame may be blown out. According to the conventional gas recirculating furnace, the recirculation effect of the recirculating gas current is not enough, and reduction in size or realization of high performance of the furnace can not be achieved.
Moreover, in case of the continuous gas recirculating furnace, the end portion of the furnace is provided on the side of the entry opening 108 in order to lower the exhaust temperature as possible. Therefore, since the full quantity of combustion gas generated for increasing the heat in the respective zones 107a, . . . , 107e is collected to the end portion of the furnace and exhausted from the exhaust opening 110, the temperature in any zone is influenced by other zones (the upper right side in FIG. 12) formed on the upstream side in the flow of the combustion gas and independence of the furnace temperature can not be maintained. The furnace temperature becomes high on the side of an exit opening 109 for the heating target W and low on the side of the entry opening 108 for the same, and the temperature can not be increased in the zone 107a, which requires the maximum heat, of the entry opening 108 for the heating target. That is, even if the temperature in the furnace on the side of the entry opening for the heating target, i.e., on the side of the exhaust opening 110 is increased to rapidly heighten the temperature, the gas is exhausted with the high temperature maintained, which results in waste of energy. Further, discharging the high-temperature exhaust gas in the air adversely influences the circumferential environment. The temperature on the inlet side can not be increased and it must be heightened gradually and slowly. As a result, the length of the furnace becomes large, thereby wasting fuel. Further, even if the exhaust temperature is tried to be decreased, it is still high and is not sufficiently low, deteriorating the thermal efficiency.
In addition, uniform heating is impossible on the both right and left sides of the heating target W (front and rear sides in the flow direction of the gas current) because a flow direction of the gas current is fixed. In case of the continuous gas recirculating furnace, heat transfer due to radiation between the heating targets in the adjacent zones causes a difference in temperature between the upstream and downstream sides in a direction for carrying in such heating targets. In other words, even if such heating targets are identical or they are put on the same tray, one heating target receives radiant heat from the other heating target having a high temperature on the upstream side, whereas the heating target having a low temperature on the downstream side takes heat, thereby producing a difference in temperature. Consequently, uniform heating is difficult.
An object of the present invention is to provide a gas recirculating furnace with the high heat transfer performance. It is another object of the present invention to provide a gas recirculating furnace being capable of assuring a large quantity of high-temperature recirculating current with low power. It is still another object of the present invention to provide a gas recirculating furnace which can form a gas current having a high and uniform temperature in the entire area in the furnace.